Fatigue is a common symptom and I have discussed it previously; look for “energy and fatigue”. Certain individuals, usually in their 30s and 40s, develop severe fatigue that is unrefreshed by sleep, and interferes with their normal activity.
Sometimes CFS is accompanied by sore throat, swollen glands, and Headache. The doctor usually finds no physical,laboratory or clinical abnormalities. If This fatigue continues on for many months, and is life-changing it has been called chronic fatigue syndrome, or CFS.
There is another similar chronic problem that is called fibromyalgia, and some authorities think of them as the same problem. To be diagnosed as CFS, The patient needs to have difficulties with memory, focus and concentration, or dizziness that worsens with moving from laying or sitting to standing.
I had had two such patients when I was in practice, Who were sent to me to rule out allergies. I found no such allergies and tried to encourage a healthy lifestyle including good sleep, diet, and exercise. My patients seem to get better with some exercise, although exercise often causes more fatigue according to the literature.
I have heard many causes postulated for this condition such as hormonal difficulties, immune system problems, or psychological factors. Infections particularly have been suspected such as cytomegalovirus, herpesviruses, or Epstein-Barr virus, but nothing consistent has shown up.
Since sleep does not restore energy, consulting a sleep center to rule out sleep apnea is reasonable. Such a condition following in the wake of COVID-19 would probably be attributed to that condition. Perhaps CFS is indeed due to an as yet undiscovered viral infection. Please check out the accompanying Mayo clinic summary.